Carbon material, material for a battery electrode, and battery

ABSTRACT

A carbon material and a material for a battery electrode which is suitable for use as an electrode material for an aqueous-electrolyte secondary battery, which material includes optical structures having a specific shape, and in which material the ratio IG/ID (R value) between the peak intensity (ID) of a peak in a range of 1300 to 1400 cm−1 and the peak intensity (IG) of a peak in a range of 1580 to 1620 cm−1 measured by Raman spectroscopy spectra when particles of the carbon material are measured with Raman microspectrometer is 0.38 or more and 1.2 or less and the average interplanar spacing d002 of plane (002) by the X-ray diffraction method is 0.335 nm or more and 0.338 nm or less; and a secondary battery excellent in charge/discharge cycle characteristics and large current load characteristics.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 14/908,281filed Jan. 28, 2016 which is a National Stage of InternationalApplication No. PCT/JP2014/069831 filed Jul. 28, 2014, claiming prioritybased on Japanese Patent Application No. 2013-157068 filed Jul. 29,2013, the contents of all of which are incorporated herein by referencein their entirety.

TECHNICAL FIELD

The present invention relates to a carbon material, a material for abattery electrode, and a battery. More specifically, the presentinvention relates to a carbon material which is suitable as an electrodematerial for a non-aqueous electrolyte secondary battery; a material fora battery electrode; and a secondary battery excellent incharge/discharge cycle characteristics and large current loadcharacteristics.

BACKGROUND ART

As a power source of a mobile device, or the like, a lithium ionsecondary battery is mainly used. The function of the mobile device orthe like is diversified, resulting in increasing in power consumptionthereof. Therefore, a lithium ion secondary battery is required to havean increased battery capacity and, simultaneously, to have an enhancedcharge/discharge cycle characteristic.

Further, there is an increasing demand for a secondary battery with ahigh output and a large capacity for electric tools such as an electricdrill and a hybrid automobile. In this field, conventionally, a leadsecondary battery, a nickel-cadmium secondary battery, and anickel-hydrogen secondary battery are mainly used. A small and lightlithium ion secondary battery with high energy density is highlyexpected, and there is a demand for a lithium ion secondary batteryexcellent in large current load characteristics.

In particular, in applications for automobiles, such as battery electricvehicles (BEV) and hybrid electric vehicles (HEV), a long-term cyclecharacteristic over 10 years and a large current load characteristic fordriving a high-power motor are mainly required, and a high volume energydensity is also required for extending a cruising distance, which aresevere as compared to mobile applications.

In the lithium ion secondary battery, generally, a lithium salt, such aslithium cobaltate, is used as a positive electrode active material, anda carboneous material, such as graphite, is used as a negative electrodeactive material.

Graphite is classified into natural graphite and artificial graphite.

Among those, natural graphite is available at a low cost. However, asnatural graphite has a scale shape, if natural graphite is formed into apaste together with a binder and applied to a current collector, naturalgraphite is aligned in one direction. When an electrode made of such amaterial is charged, the electrode expands only in one direction, whichdegrades the performance of the electrode. Natural graphite, which hasbeen granulated and formed into a spherical shape, is proposed, however,the resulting spherical natural graphite is aligned because of beingcrushed by pressing in the course of electrode production. Further, thesurface of the natural graphite is active, and hence a large amount ofgas is generated during initial charging, which decreases an initialefficiency and degrades a cycle characteristic. In order to solve thoseproblems, Japanese Patent publication No. 3534391 (U.S. Pat. No.6,632,569, Patent Document 1), etc. propose a method involving coatingartificial carbon on the surface of the natural graphite processed intoa spherical shape.

Regarding artificial graphite, there is exemplified a mesocarbonmicrosphere-graphitized article described in JP 04-190555 A (PatentDocument 2) and the like.

Artificial graphite typified by graphitized articles made of oil, coalpitch, coke and the like is available at a relatively low cost. However,a needle-shaped coke with high crystallinity is scaly and tends to bealigned. In order to solve this problem, the method described inJapanese patent publication No. 3361510 (Patent Document 3) and the likeyield results.

Further, negative electrode materials using so-called hard carbon andamorphous carbon described in JP 07-320740 A (U.S. Pat. No. 5,587,255;Patent Document 4) are excellent in a characteristic with respect to alarge current and also have a relatively satisfactory cyclecharacteristic.

JP 2003-77534 A (Patent Document 5) teaches that excellent high-ratedischarge can be achieved by using artificial graphite havinghighly-developed fine pores.

WO 2011/049199 (U.S. Pat. No. 8,372,373; Patent Document 6) disclosesartificial graphite being excellent in cycle characteristics.

JP 2002-270169 A (U.S. Pat. No. 7,141,229; Patent Document 7) disclosesan artificial graphite negative electrode produced from needle-shapedgreen coke having anisotropy based on a flow configuration texture.

WO 2003/064560 (U.S. Pat. No. 7,323,120; JP-A-2005-515957; PatentDocument 8) discloses an artificial graphite negative electrode producedfrom cokes coated with petroleum pitch in a liquid phase.

PRIOR ART Patent Documents

Patent Document 1: JP 3534391 B2

Patent Document 2: JP 04-190555 A

Patent Document 3: JP 3361510 B2

Patent Document 4: JP 07-320740 A

Patent Document 5: JP 2003-77534 A

Patent Document 6: WO 2011/049199

Patent Document 7: JP 2002-270169 A

Patent Document 8: WO 2003/064560 (JP 2005-515957 A)

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The material produced by the method described in Patent Document 1 canaddress a high-capacity, a low-current, and an intermediate-cyclecharacteristic required by the mobile applications, etc. However, it isvery difficult for the material to satisfy the requests such as a largecurrent and an ultralong-term cycle characteristic of a large battery asdescribed above.

The graphitized article described in Patent Document 2 is awell-balanced negative electrode material, and is capable of producing abattery with a high capacity and a large current. However, it isdifficult to achieve the cycle characteristic for a much longer periodof time than the one for mobile applications, which are required for alarge battery.

The method according to Patent Document 3 can allow the use of not onlyfine powder of an artificial graphite material but also fine powder of anatural graphite, or the like, and exhibits very excellent performancefor a negative electrode material for the mobile applications. Thismaterial can address the high-capacity, the low-current, and theintermediate cycle characteristic required for the mobile applications,etc. However, this material has not satisfied requests such as a largecurrent and an ultralong-term cycle characteristic of a large battery asdescribed above.

The volume energy density of the negative electrode material describedin Patent Document 4 is too low and the price of the material is veryexpensive, and thus, such negative electrode materials are only used forsome special large batteries.

In Patent Document 5, the capacity retention at the time of charge anddischarge is not sufficient for actual use in secondary batteries.

In Patent Document 6, the graphite has a highly dense texture and therewas room for improvement on the diffusion of the active material ion.

In Patent Document 7, although the capacity and initial charge-dischargeefficiency showed some improvement compared to the case of usingconventional artificial graphite, the graphite negative electrode hasnot been developed to a practical level.

In Patent Document 8, the electrode capacity density has remained as anissue to be solved. Also, the production involves an operation of usinglarge quantities of organic solvent and evaporating it, which makes theproduction method cumbersome.

Means to Solve the Problem

-   [1] A carbon material, wherein the ratio I_(G)/I_(D) (R value)    between the peak intensity (I_(D)) of a peak in a range of 1300 to    1400 cm⁻¹ and the peak intensity (I_(G)) of a peak in a range of    1580 to 1620 cm⁻¹ measured by Raman spectroscopy spectra when    particles of the carbon material are measured with Raman    microspectrometer is 0.38 or more and 1.2 or less and the average    interplanar spacing d002 of plane (002) by the X-ray diffraction    method is 0.335 nm or more and 0.338 nm or less; and by observing    the optical structures in the cross-section of the formed body made    of the carbon material in a rectangular field of 480 μm×640 μm under    a polarizing microscope, when areas of the optical structures are    accumulated from a smallest structure in an ascending order, SOP    represents an area of an optical structure whose accumulated area    corresponds to 60% of the total area of all the optical structures;    when the structures are counted from a structure of a smallest    aspect ratio in an ascending order, AROP represents the aspect ratio    of the structure which ranks at the position of 60% in the total    number of all the structures; and when D50 represents a volume-based    average particle diameter by laser diffraction method; SOP, AROP and    D50 satisfy the following relationship:

1.5≤AROP≤6 and

0.2×D50≤(SOP×AROP)^(1/2)<2×D50.

-   [2] The carbon material as described in [1] above, a main component    of which is artificial graphite.-   [3] The carbon material as described in [1] or [2] above, wherein    the carbon material has a volume-based average particle diameter by    laser diffraction method (D50) of 1 μm or more and 50 μm or less.-   [4] The carbon material as described in any one of [1] to [3] above,    BET specific surface area of which is 2 m²/g or more and 25 m²/g or    less.-   [5] A method for producing the carbon material described in any one    of [1] to [4] above, comprising a process of subjecting the    particles obtained by pulverizing the calcined coke to heat    treatment at a temperature of 2,400° C. or more and 3,600° C. or    less, mixing the resultant with the particles obtained by    pulverizing petroleum pitch or coal-tar pitch, and subjecting the    mixture to heat treatment at a temperature of 800° C. or more and    1,400° C. or less.-   [6] The production method as described in [5] above, comprising    pulverization or crushing the mixture after the process of the heat    treatment of 800° C. or more and 1,400° C. or less.-   [7] The production method as described in [5] or [6] above, wherein    a volume-based average particle diameter by laser diffraction method    of the particles obtained by pulverizing calcined coke (D50) Dc is 1    μm or more and 50 μm or less, and a volume-based average particle    diameter of the particles obtained by pulverizing petroleum pitch or    coal-tar pitch (D50) Dp is smaller than Dc and is 0.01 μm or more    and 25 μm or less.-   [8] The production method as described in [7] above, wherein Dc/Dp    is 1.5 or more and less than 200.-   [9] The production method as described in any one of [5] to [8]    above, wherein the mass of the particles obtained by pulverizing    petroleum pitch or coal-tar pitch is 5 mass % or more and 35 mass %    or less to the total mass of the particles obtained by pulverizing    calcined coke and the particles obtained by pulverizing petroleum    pitch or coal-tar pitch.-   [10] The production method as described in any one of [5] to [9]    above, wherein by observing the optical structures of the calcined    coke in a rectangular field of 480 μm×640 μm under a polarizing    microscope, when areas of the optical structures are accumulated    from a smallest structure in an ascending order, an area of an    optical structure whose accumulated area corresponds to 60% of the    total area of all the optical structures is 10 μm² or more and 5,000    μm² or less; and when the optical structures are counted from a    structure of a smallest aspect ratio in an ascending order, the    aspect ratio of the structure which ranks at the position of 60% in    the total number of all the structures is 1.5 or more and 6 or less.-   [11] A material for a battery electrode, comprising the carbon    material described in any one of [1] to [4].-   [12] A carbon material for a battery electrode, comprising 100 parts    by mass of the carbon material described in any one of [1] to [4]    and 0.01 to 200 parts by mass of natural graphite or artificial    graphite, wherein an average interplanar spacing (d002) of the    natural graphite or artificial graphite is 0.3380 nm or less.-   [13] A material for a battery electrode, comprising 100 parts by    mass of the carbon material described in any one of [1] to [4] and    0.01 to 120 parts by mass of natural graphite or artificial    graphite, wherein an aspect ratio of the natural graphite or    artificial graphite is 2 to 100, and an average interplanar spacing    (d002) of the natural graphite or artificial graphite is 0.3380 nm    or less.-   [14] A paste for an electrode comprising the carbon material for a    battery electrode described in any one of [11] to [13] above and a    binder.-   [15] An electrode comprising a formed body of the paste for an    electrode described in [14] above.-   [16] A battery comprising the electrode described in [15] above as a    constituting element.

Effects of the Invention

Using the carbon material of the present invention as the material forthe battery electrode improves the diffusion of lithium ions, andtherefore a battery electrode can be obtained which has a high energydensity and the capability of high-speed charge and discharge when ansecondary battery is fabricated, while maintaining a high capacity, theinitial high coulomb efficiency and the high cycle characteristics.

Further, the carbon material of the present invention can be produced bythe method excellent in economic efficiency and mass productivity withsafety improved.

MODE FOR CARRYING OUT THE INVENTION (1) Carbon Material

The electrode of the rechargeable battery is required to charge moreelectricity per unit volume. The graphite used as an electrode activesubstance of the lithium secondary battery is excellent in coulombefficiency at initial charge and discharge. However, there is an upperlimit to the stoichiometric proportion of the lithium atom insertion tocarbon atoms and it is difficult to increase the energy density per massto the stoichiometric proportion or higher. Therefore, it is necessaryto increase the mass per electrode volume: i.e. the electrode density toimprove the energy density of the electrode.

Generally, an electrode for a battery is produced by drying an activesubstance applied onto a current collector plate and subsequentpressing. Pressing improves the filling property of the active substanceper volume, and if the active substance is soft enough to be deformed tosome degree by pressing, it is possible to significantly increase theelectrode density.

Since graphite particles are hard when the graphite has a complicatedstructure or low orientation, it is necessary to allow the graphiteparticles to have a large structure in order to increase the electrodedensity. It has been long known that there is a structure which exhibitsoptical anisotropy by crystals developed and graphite planes arranged,and a structure which exhibits optical isotropy by crystals notdeveloped completely or largely disordered such as hard carbon. Withrespect to the observation of these structures, a crystal size can bemeasured by the X-ray diffraction method and the structures can beobserved by a polarizing microscope observation method described in, forexample, “Modern Carbon Material Experimental Technology (Analysis part)edited by The Carbon Society of Japan (2001), published by SipecCorporation, pages 1-8”. In the present invention, a structure in whichpolarization can be observed is referred to as an optical structure.

In the carbon material in a preferable embodiment of the presentinvention, the size and shape of the optical structures are within aspecific range. Furthermore, due to an appropriate degree ofgraphitization, it becomes a material being excellent both in easinessto be collapsed as a material for an electrode and in batteryproperties.

With respect to the size and shape of the optical structure, it isdesirable that the above-mentioned carbon material satisfies thefollowing formula:

1.5≤AROP≤6 and

0.2×D50≤(SOP×AROP)^(1/2)<2×D50

By observing optical structures in the cross-section of the molded bodymade of the carbon material in a rectangular field of 480 μm×540 μmunder a polarizing microscope, when areas of the optical structures areaccumulated from the smallest structure in an ascending order, SOPrepresents the area of the optical structure whose accumulated areacorresponds to 60% of the total area of all the optical structures. Whenthe structures are counted from a structure of the smallest aspect ratioin an ascending order, AROP represents the aspect ratio of the structurewhich ranks at the position of 60% in the total number of all thestructures.

D50 represents a particle diameter corresponding to the accumulateddiameter of 50% of the cumulative total of diameters (an averageparticle diameter) based on a volume measured by laser-diffractmetryparticle size distribution analyzer, and represents an apparent diameterof the scale-like particles. As a laser diffraction type particle sizedistribution analyzer, for example, Mastersizer (registered trademark)produced by Malvern Instruments Ltd. or the like can be used.

The carbon material in a preferable embodiment of the present inventionhas a scale-like shape. Since the optical structures in the carbonmaterial are cured while flowing, it is often strip-shaped. When thecross-section of a formed body composed of the carbon material isobserved, the shape of the optical structures is almost rectangular, andit can be assumed that the area of the structure corresponds to theproduct of the longer diameter and the shorter diameter of thestructure. Also, the shorter diameter is the longer diameter/aspectratio. Assuming that the optical structure as an object to be measuredfor the area represented by SOP and the optical structure as an objectto be measured for the aspect ratio represented by AROP are the same,the longer diameter in the optical structure turns to be(SOP×AROP)^(1/2). That is, (SOP×AROP)^(1/2) defines the longer diameterin an optical structure having a specific size, and based on the ratioof (SOP×AROP)^(1/2) to the average particle diameter (D50), theabove-mentioned formula defines that the optical structure is largerthan a certain size.

(SOP×AROP)^(1/2) which defines a longer diameter of an optical structureis generally smaller than an average particle diameter D50. However,when the (SOP×AROP)^(1/2) value is closer to D50, it means that theparticles in the carbon material consist of a smaller number of opticalstructures. In a case where (SOP×AROP)^(1/2) is smaller compared to D50,it means that the particles in the carbon material comprise a largenumber of optical structures. When the (SOP×AROP)^(1/2) value is 0.2×D50or more, there are fewer borders of the optical structures, which ispreferable for the lithium ion diffusion and enables a high-rate chargeand discharge. When the value is larger, the carbon material can retaina larger number of lithium ions. The value is preferably 0.25×D50 ormore, more preferably 0.28×D50 or more, and still more preferably0.35×D50 or more. The value is less than 2×D50 at maximum, andpreferably 1×D50 or less.

The average particle diameter (D50) of the carbon material in apreferable embodiment of the present invention is 1 μm or more and 50 μmor less. Pulverizing by special equipment is required to make D50 lessthan 1 μm and more energy is required as a result. On the other hand, ifthe D50 value is too large, it takes a longer time for the lithiumdiffusion in the negative electrode material and it tends to reduce thecharge and discharge rate. A preferred D50 value is from 5 μm to 35 μm.Considering that fine powder has a large surface area and is likely togive rise to an unintended reaction so that it should be reduced, D50 ismore preferably 10 μm or more. When the carbon material is for use inthe driving power source for automobile and the like which requiresgenerating a large current, D50 is preferably 25 μm or less.

The aspect ratio of the carbon material, AROP, is from 1.5 to 6, morepreferably from 2.0 to 4.0. An aspect ratio larger than the above lowerlimit is preferable because it allows the optical structures to slideover each other and an electrode having a high density can be easilyobtained. An aspect ratio smaller than the upper limit is preferablebecause it requires less energy to synthesize a raw material.

The methods for observation and analysis of the optical structures areas described below.

[Production of Polarizing Microscope Observation Sample]

The “cross-section of the formed body made of a carbon material” as usedherein is prepared as follows.

A double-stick tape is attached to the bottom of a sample container madeof plastic with an internal volume of 30 cm³, and two spatula scoops(about 2 g) of a sample for observation is placed on the double-sticktape. A curing agent (Curing Agent (M-agent) (trade name), produced byNippon Oil and Fats Co., Ltd., available from Marumoto Struers K.K.) isadded to cold mounting resin (Cold mounting resin #105 (trade name),produced by Japan Composite Co., Ltd., available from Marumoto StruersK.K.), and the mixture is kneaded for 30 seconds. The resultant mixture(about 5 ml) is poured slowly to the sample container to a height ofabout 1 cm and allowed to stand still for 1 day to be coagulated. Next,the coagulated sample is taken out and the double-stick tape is peeledoff. Then, a surface to be measured is polished with a polishing machinewith a rotary polishing plate.

The polishing is performed so that the polishing surface is pressedagainst the rotary surface. The polishing plate is rotated at 1,000 rpm.The polishing is performed successively, using #500, #1000, and #2000 ofthe polishing plates in this order, and finally, mirror-surfacepolishing is performed, using alumina (BAIKALOX (registered trademark)type 0.3CR (trade name) with a particle diameter of 0.3 μm, produced byBAIKOWSKI, available from Baikowski Japan).

The polished sample is fixed onto a preparation with clay and observedwith a polarizing microscope (BX51, produced by Olympus Corporation).

[Polarizing Microscope Image Analysis Method]

The observation was performed at 200-fold magnification. An imageobserved with the polarizing microscope is photographed by connecting aCAMEDIA C-5050 ZOOM digital camera produced by Olympus Corporation tothe polarizing microscope through an attachment. The shutter time is 1.6seconds. Among the photographing data, images with 1,200×1,600 pixelswere included in the analysis. It corresponds to investigation in amicroscope field of 480 μm×540 μm. The image analysis was performedusing ImageJ (produced by National Institutes of Health) to judge blueportions, yellow portions, magenta portions and black portions.

The parameters defining each color when ImageJ was used are given below.

TABLE 1 Hue value Saturation value Brightness valule Blue 150 to 190 0to 255 80 to 255 Yellow 235 to 255 0 to 255 80 to 255 Magenta 193 to 255180 to 255  120 to 255  Black  0 to 255 0 to 255  0 to 120

The statistical processing with respect to the detected structures isperformed using an external macro-file. The black portions, that is,portions corresponding not to optical structures but to resin areexcluded from the analysis, and the area and aspect ratio of each ofblue, yellow and magenta optical structures are to be calculated.

As mentioned above, examples of a negative electrode material, which hasrelatively large structures and a small crystal interplanar spacing(d002) to be described later, include natural graphite. Natural graphitebecomes scale-like by pulverization. However, there are many defects onthe edge surface of the pulverized natural graphite particles. Incontrast, the carbon material of the present invention is characterizedin that defects on the edge surface of the particles are not exposed,and the material is excellent in battery properties. Examples of themethod for confirming the crystallinity on the surface of the particlesinclude Raman spectrometry.

The Raman spectrum can be measured by observation using, for example,NRS-5100 produced by JASCO Corporation.

In the measurement by a Raman spectrum, the peak in a range of 1300 to1400 cm⁻¹ is based on sp3 bonds, and the peak intensity in the range(I_(D)) indicates the abundance of defects on the surface of carbonparticles. The peak in a range of 1580 to 1620 cm⁻¹ is based on sp2bonds and shows the abundance of the bonds derived from graphite on theparticle surface.

The carbon material in a preferable embodiment of the present inventionis characterized in having an R value, that is (I_(G)/I_(D)), highercompared to the particles obtained by graphitization after pulverizingthe calcined coke. Specifically, when the particles of the carbonmaterial is measured by Raman spectrometer, the intensity ratioI_(G)/I_(D) (R value) between the peak intensity (I_(D)) in a range of1300 to 1400 cm⁻¹ and the peak intensity (I_(G)) in a range of 1580 to1620 cm⁻¹ observed by Raman spectroscopy spectra is from 0.38 or moreand 1.2 or less. R value is more preferably from 0.38 or more and 1.0 orless and still more preferably from 0.4 or more and 0.8 or less. Whenthe R value is ether too large or too small, it promotes side reactionsat the time of charge and discharge by the existence of many defects. Byallowing the carbon material to have an appropriate R value, it becomesa material which undergoes less self-discharge and degradation of abattery when it is held after charging.

The carbon material in a preferable embodiment of the present inventionhas an average interplanar distance (002) by the X-ray diffractionmethod of 0.338 nm or less. This increases the amount of lithium ions tobe intercalated and desorbed; i.e. increases the weight energy density.Further, a thickness Lc of the crystal in the C-axis direction ispreferably 50 to 1,000 nm from the viewpoint of the weight energydensity and easiness to be collapsed. When d002 is 0.338 nm or less,most of the optical structures observed by a polarizing microscope arefound to be optically anisotropic.

It is difficult to manufacture a carbon material having d002 of lessthan 0.335 nm, and a carbon material having d002 of 0.335 nm or more isindustrially useful.

d002 and Lc can be measured using a powder X-ray diffraction (XRD)method by a known method (see I. Noda and M. Inagaki, Japan Society forthe Promotion of Science, 117th Committee material, 117-71-A-1 (1963),M. Inagaki et al., Japan Society for the Promotion of Science, 117thcommittee material, 117-121-C-5 (1972), M. Inagaki, “carbon”, 1963, No.36, pages 25-34).

In the preferred embodiment of the present invention, as pulverizationis not performed after graphitization, a rhombohedral peak ratio is 5%or less, more preferably 1% or less.

When the graphite material falls in such ranges, an interlayer compoundwith lithium is formed smoothly. If the interlayer compound is used as anegative electrode material in a lithium secondary battery, the lithiumocclusion/release reaction is hardly inhibited, which enhances a rapidcharging/discharging characteristic.

It should be noted that the peak ratio (x) of the rhombohedral structurein graphite powder is obtained from actually measured peak strength (P1)of a hexagonal structure (100) plane and actually measured peak strength(P2) of a rhombohedral structure (101) plane by the followingexpression.

x=P2/(P1+P2)

In a preferred embodiment of the present invention, the BET specificsurface area of the carbon material is 2 m²/g or more and 25 m²/g orless, more preferably 4 m²/g or more and 20 m²/g or less, and still morepreferably 8 m²/g or more and 15 m²/g or less. By setting the BETspecific surface area to be within the above-mentioned range, a widearea to be contacted with an electrolyte can be secured withoutexcessive use of a binder, and thereby lithium ions can be smoothlyintercalated and released, and the reaction resistance of the batterycan be lowered.

The BET specific surface area is measured by a common method ofmeasuring the absorption and desorption amount of gas per mass. As ameasuring device, for example, NOVA-1200 can be used.

It is preferred that the loose bulk density (0 tapping) of the carbonmaterial of the present invention be 0.3 g/cm³ or more, and the powderdensity (tap density) when tapping is performed 400 times be 0.4 g/cm³or more and 1.5 g/cm³ or less. The powder density is more preferably0.45 g/cm³ and 1.4 g/cm³ or less, and most preferably 0.5 g/cm³ or moreand 1.3 g/cm³ or less.

The loose bulk density is obtained by dropping 100 g of the sample to agraduated cylinder from a height of 20 cm, and measuring the volume andmass without applying a vibration. The tap density is obtained bymeasuring the volume and mass of 100 g of powder tapped 400 times usingan Autotap produced by Quantachrome Instruments.

These methods are based on ASTM B527 and JIS K5101-12-2, and the fallheight of the Autotap in the tap density measurement is 5 mm.

By setting the loose bulk density to be 0.3 g/cm³ or more, the electrodedensity before pressing at a time of application to an electrode can beenhanced further. Based on this value, it can be predicted whether ornot a sufficient electrode density can be obtained by one roll pressing.Further, if the tap density is within the above-mentioned range, theelectrode density achieved during pressing can be enhanced sufficiently.

The carbon material in a preferred embodiment of the present inventionmay be the one in which a part of carbon fiber adheres to the surfacethereof. By allowing a part of the carbon fiber to adhere to the surfaceof the carbon material, the carbon fiber in an electrode is easilydispersed, and the cycle characteristic and the current loadcharacteristic are further enhanced due to the synergetic effect of thecarbon fiber in combination with the characteristics of the carbonmaterial serving as the core material.

Although the adhesion amount of the carbon fiber is not particularlylimited, the adhesion amount is preferably 0.1 to 5 parts by mass interms of 100 parts by mass of the carbon material serving as a core.

Examples of the carbon fiber include: organic carbon fiber such asPAN-based carbon fiber, pitch-based carbon fiber, and rayon-based carbonfiber; and vapor-grown carbon fiber. Of those, particularly preferred isvapor-grown carbon fiber having high crystallinity and high heatconductivity. In the case of allowing the carbon fiber to adhere to thesurfaces of the graphite particles, particularly preferred isvapor-grown carbon fiber.

Vapor-grown carbon fiber is, for example, produced by: using an organiccompound as a material; introducing an organic transition metal compoundas a catalyst into a high-temperature reaction furnace with a carriergas to form fiber; and then conducting heat treatment (see, for example,JP 60-54998 A and JP 2778434 B2). The vapor-grown carbon fiber has afiber diameter of 2 to 1,000 nm, preferably 10 to 500 nm, and has anaspect ratio of preferably 10 to 15,000.

Examples of the organic compound serving as a material for carbon fiberinclude toluene, benzene, naphthalene, ethylene, acetylene, ethane,natural gas, a gas of carbon monoxide or the like, and a mixturethereof. Of those, an aromatic hydrocarbon such as toluene or benzene ispreferred.

The organic transition metal compound includes a transition metalserving as a catalyst. Examples of the transition metal include metalsof Groups IVa, Va, VIa, VIIa, and VIII of the periodic table. Preferredexamples of the organic transition metal compound include compounds suchas ferrocene and nickelocene.

The carbon fiber may be obtained by pulverizing or shredding long fiberobtained by vapor deposition or the like. Further, the carbon fiber maybe agglomerated in a flock-like manner.

Carbon fiber which has no pyrolyzate derived from an organic compound orthe like adhering to the surface thereof or carbon fiber which has acarbon structure with high crystallinity is preferred.

The carbon fiber with no pyrolyzate adhering thereto or the carbon fiberhaving a carbon structure with high crystallinity can be obtained, forexample, by sintering (heat-treating) carbon fiber, preferably,vapor-grown carbon fiber in an inactive gas atmosphere. Specifically,the carbon fiber with no pyrolyzate adhering thereto is obtained by heattreatment in inactive gas such as argon at about 800° C. to 1,500° C.Further, the carbon fiber having a carbon structure with highcrystallinity is obtained by heat treatment in inactive gas such asargon preferably at 2,000° C. or more, more preferably 2,000° C. to3,000° C.

It is preferred that the carbon fiber contains branched fiber. Further,the fiber as a whole may have a portion having hollow structurescommunicated with each other. For this reason, carbon layers forming acylindrical portion of the fiber are formed continuously. The hollowstructure refers to a structure in which a carbon layer is rolled up ina cylindrical shape and includes an incomplete cylindrical structure, astructure having a partially cut part, two stacked carbon layersconnected into one layer, and the like. Further, the cross-section isnot limited to a complete circular cross-section, and the cross-sectionof the cylinder includes an oval cross-section or a polygonalcross-section.

Further, the average interplanar spacing d002 of a (002) plane by theX-ray diffraction method of the carbon fiber is preferably 0.344 nm orless, more preferably 0.339 nm or less, particularly preferably 0.338 nmor less. Further, it is preferred that a thickness (L_(c)) in a C-axisdirection of crystal is 40 nm or less.

(2) Method for Producing a Carbon Material

The carbon material in a preferable embodiment of the present inventioncan be produced by subjecting the particles obtained by pulverizingcalcined coke to heat treatment at a temperature of 2,400° C. or higherand 3,600° C. or lower, mixing the resultant and the particles obtainedby pulverizing petroleum pitch or coal-tar pitch, and subjecting themixture to heat treatment at a temperature of 800° C. or higher and1,400° C. or lower.

As a raw material of calcined coke, for example, petroleum pitch, coalpitch, coal pitch coke, petroleum coke and the mixture thereof can beused. Among these, preferred is the coke obtained by a delayed cokingprocess under specific conditions and the subsequent heating under aninert atmosphere.

Examples of raw materials to pass through a delayed coker include decantoil which is obtained by removing a catalyst after the process of fluidcatalytic cracking to heavy distillate at the time of crude refining,and tar obtained by distilling coal tar extracted from bituminous coaland the like at a temperature of 200° C. or more and heating it to 100°C. or more to impart sufficient flowability. It is desirable that theseliquids are heated to 450° C. or more, or even 510° C. or more, duringthe delayed coking process, at least at an inlet of the coking drum inorder to increase the residual carbon ratio of the coke at the time ofcalcination. Also, pressure inside the drum is kept at preferably anordinary pressure or higher, more preferably 300 kPa or higher, stillmore preferably 400 kPa or higher to increase the capacity of a negativeelectrode. As described above, by performing coking under more severeconditions than usual, the reaction of the liquids is further enhancedand coke having a higher degree of polymerization can be obtained.

The obtained coke is to be cut out from the drum by water jetting, androughly pulverized to lumps about the size of 5 centimeters with ahammer and the like. A double roll crusher and a jaw crusher can be usedfor the rough pulverization, and it is desirable to pulverize the cokeso that the particles larger than 1 mm in size account for 90 mass % ormore of the powder. If the coke is pulverized too much to generate alarge amount of fine powder having a diameter of 1 mm or less, problemssuch as the dust stirred up after drying and the increase in burnoutsmay arise in the subsequent processes such as heating.

Next, the roughly pulverized coke is subjected to calcination. Thecalcination means to perform heating to remove moisture and organicvolatile components.

The coke before calcination is relatively flammable. Therefore, the cokeis to be soaked with water to prevent fires. The coke soaked with watercontaminates equipment and surrounding space with muddy fine powdercontaining water, and is inferior in handleability. Calcination canprovide significant advantage in terms of handleability. Also, when thecalcined coke is subjected to graphitization, it promotes thedevelopment of crystals.

The calcination is performed by electric heating and flame heating ofLPG, LNG, heating oil and heavy oil. Since a heat source of 2,000° C. orless is sufficient to remove moisture and organic volatile components,flame heating as an inexpensive heat source is preferable for massproduction. When the treatment is performed on a particularly-largescale, energy cost can be reduced by an inner-flame or inner-heatingtype heating of coke while burning fuel and the organic volatilecomponents contained in the unheated coke in a rotary kiln.

It is desirable that the area and aspect ratio of a specific opticalstructure of the calcined coke are within a specific range. The area andaspect ratio of an optical structure can be calculated by theabove-mentioned method. Also, when the calcined coke is obtained as alump of a few centimeters in size, the lump as produced is embedded inresin and subjected to mirror-like finishing and the like, and thecross-section is observed by a polarizing microscope to calculate thearea and aspect ratio of an optical structure.

In the case where the optical structures are observed in a rectangularfield of 480 μm×640 μm in the cross-section of the calcined coke under apolarizing microscope, when areas of the optical structures areaccumulated from the smallest structure in an ascending order, an areaof an optical structure whose accumulated area corresponds to 60% of thetotal area of all the optical structures is preferably 10 μm² to 5,000μm², more preferably 10 μm² to 1,000 μm², and still more preferably 20μm² to 500 μm². When the calcined coke having the area of an opticalstructure within the above-mentioned range is graphitized, the graphiteis going to have a fully developed crystal structure and can retainlithium ions at a higher density. Also, as the crystals develop in amore aligned state and the fracture surfaces of the crystals slide overeach other, the resultant graphite has a higher degree of freedom forthe particle shape when an electrode is pressed, which improves fillingproperty and is preferable.

In the case where the optical structure of the calcined coke is observedin the same way as described above, when the optical structures arecounted from a structure of the smallest aspect ratio in an ascendingorder, the aspect ratio of the structure which ranks at the position of60% in the total number of all the structures is preferably 1.5 to 6.

Next, the calcined coke is to be pulverized.

There is not particular limit to the method of pulverization, andpulverization can be performed using a known jet mill, hammer mill,roller mill, pin mill, vibration mill or the like.

It is desirable to perform pulverization so that coke has a volume-basedaverage particle diameter (D50) of from 1 μm to 50 μm. To performpulverization to make D50 less than 1 μm, it requires use of specificequipment and a large amount of energy. When D50 is too large, thelithium ion diffusion takes time when the coke is made into an electrodeand it is likely to reduce the charge and discharge rate. D50 is morepreferably from 5 μm to 35 μm. Considering that fine powder ispreferably reduced because it has a large surface area and is likely togive rise to an unintended reaction, D50 is more preferably 10 μm ormore. When the carbon material is for use in the driving power sourcefor automobile and the like required generating a large current, D50 ispreferably 25 μm or less.

In the pulverized calcined coke, lattice defects are generated on thefracture face parallel to c-axis (particle edge surface). When the cokeis graphitized with the lattice defects left in it, many highly-reactivedefects will be generated on the particle edge surface.

When there are many defects on the particle edge surface, the particlesbecome highly responsive to the electrolyte composition and consume muchelectricity in the initial charge: i.e. at the time of the lithium ionintercalation, which results in forming an excessively thick coating. Asa result, it inhibits reversible lithium intercalation and releasereaction of lithium ions and may adversely affect on the battery lifesuch as cycle characteristics. Therefore, in the graphite edge surface,the status having fewer defects is desirable.

In order to reduce the influence of defects generated at the time ofheat treatment of the pulverized calcined coke, the particles obtainedby pulverizing a defect repairing material selected from petroleum pitchor coal-tar pitch may be mixed into the coke after graphitization, andthe mixture can be subjected to heat treatment. By performing theoperation, the defects on the fracture surface (particle edge surface)generated by the pulverization are repaired through the heat treatmentand a material in which few defects are exposed on the particle edgesurface can be obtained.

Mixing of the particles obtained by pulverizing the calcined coke andsubjecting it to heat treatment, and the above-mentioned particlesobtained by pulverizing a defect repairing material may be mixed eitherby a wet method or a dry method.

When the mixing is performed by a wet method, for example, the defectrepairing material is dissolved or dispersed in a solvent and afteradding the calcined coke which was subjected to graphitization there to,the solvent can be removed by drying. Note that an organic solvent isused in a wet method, which requires careful handling, and it isnecessary to prevent the solvent from evaporation and to collect thesolvent. Therefore, it is desirable to perform the mixing in a drymethod in which a solvent is not used.

When the mixing is performed in a dry method, it is desirable to performthe mixing with a certain force that will hardly pulverize the particlesobtained by pulverizing the calcined coke and subjecting it to heattreatment in order to make sure that the particles obtained bypulverizing calcined coke and subjecting it to heat treatment and theparticles obtained by the defect repairing material are fully mixed. Formixing, in addition to a mixer having a small pulverizing power such asa planetary and centrifugal mixer, a planetary mixer and a Henschelmixer, a mixer with a detuned pulverization performance by controllingthe liner part, blades and number of rotations of a hammer mill, aimpeller mill and the like can be suitably used. Among these, a hammermill and an impeller mill have a high mixing power and suitable forperforming a dry-method coating continuously in a short time. In mixingby a dry method, a smooth film owing to the defect repairing material isnot formed in some cases. However, the defect repairing material issoftened by the heating after mixing, spreads over the surface of theparticles obtained by pulverizing the calcined coke and subjecting it toheat treatment, and becomes a smooth film.

The average particle diameter based on a volume by laser diffractionmethod of the particles obtained by pulverizing petroleum pitch orcoal-tar pitch (D50) is smaller than that of the particles obtained bypulverizing calcined coke (D50) and is preferably 0.01 μm to 25 μm.Making the particle diameter of the defect repairing materialexcessively small not only causes the agglomeration of particles butalso could cause dust explosion. D50 is more preferably 0.5 μm or moreand still more preferably 1.0 μm or more. To make the formed film moreuniform and denser, D50 is preferably 10 μm or less and more preferably5 μm or less.

When Dc represents the average particle diameter (D50) of the particlesobtained by pulverizing calcined coke and Dp represents the averageparticle diameter (D50) of the particles obtained by pulverizingpetroleum pitch or coal-tar pitch, setting Dc/Dp value from 1.5 or moreand less than 200 enables forming a more uniform film and is desirable.When the Dc/Dp value is too large, special equipment and a large amountof energy are required to prepare extremely small defect particles of adefect repairing material, and furthermore, there may be a decrease inthe defect-repairing performance due to a decrease in an amount of theparticles of a defect repairing material to be deposited. Dc/Dp ispreferably 50 or less and more preferably 15 or less. Considering thebalance between the amount of the particles obtained by pulverizing thecalcined coke and the amount of the particles of the defect repairingmaterial to be deposited thereon, Dc/Dp is preferably 3 or more and morepreferably 8 or more.

The compounding ratio of the particles obtained by pulverizing thedefect repairing material is preferably 5 mass % or more and 35 mass %or less of the total mass of the particles obtained by pulverizing thecalcined coke and subjecting it to heat treatment and the particlesobtained by pulverizing the defect repairing material from the viewpointof the volume energy density of an electrode. The compounding ratio ismore preferably 10 mass % or more and 30 mass % or less from theviewpoint of high-rate charge and discharge, and still more preferably15 mass % or more and 25 mass % or less from the viewpoint of the weightenergy density.

After performing the dry method mixing as mentioned above in thecalcined coke subjected to heat treatment after being pulverized, thesurface of the coke can be carbonized by heating. In the case where adefect repairing material is not blended, defects remain on the surfaceof the calcined coke subjected to heat treatment after being pulverized.In contrast, in the case where the calcined coke is carbonized afterbeing mixed with a defect repairing material, the defects existing onthe particle edge surface and the like are repaired and a materialhaving few defects can be obtained, although details of the mechanismare not known.

As the major portion of particles of the obtained carbon material has ahigh degree of crystallinity, good filling property in the electrode aswell as the property inhibiting side reactions due to few defects on aparticle edge surface can be achieved, which have been hard to balance.

Graphitization is performed at a temperature of 2,400° C. or higher,more preferably 2,800° C. or higher, and still more preferably 3,050° C.or higher, and the most preferably 3,150° C. or higher. The treatment ata higher temperature further promotes the development of the graphitecrystals and an electrode having a higher storage capacity of lithiumion can be obtained. On the other hand, if the temperature is too high,it is difficult to prevent the sublimation of the carbon material and anunduly large amount of energy is required. Therefore, the graphitizationis preferably 3,600° C. or lower.

It is desirable to use electric energy to attain the above temperature.Electric energy is more expensive than other heat source and inparticular to attain a temperature of 2,000° C. or higher, an extremelylarge amount of electricity is consumed. Therefore, it is preferable notto consume the electric energy except for graphitization, and to calcinethe carbon material prior to the graphitization to remove the organicvolatile components: i.e. to make the fixed carbon content be 95% ormore, preferably 98% or more, and still more preferably 99% or more.

The graphitization treatment is conventionally carried out underatmosphere without containing oxygen, for example, in a nitrogen-sealedenvironment and an argon-sealed environment. In contrast, in the presentinvention, it is preferable to perform the graphitization treatment inan environment with a certain concentration of oxygen.

There is no limitation on the graphitization treatment as long as it isperformed in an environment with a certain oxygen concentration. Thetreatment can be carried out, for example, by a method of putting amaterial to be graphitized in a graphite crucible without closing thelid in an Acheson furnace filled with a filler of carbon particles orgraphite particles; and generating heat by passing a current through thecarbon material in a state that the top of the material is in contactwith an oxygen-containing gas to thereby carry out graphitization. Inthis case, in order to prevent the substances contained in the materialto be graphitized from reacting explosively, or to prevent theexplosively-reacted materials from being blown off, the crucible may belightly shut off from the oxygen-containing gas by covering the top ofthe crucible with a carbonized or graphitized felt and porous plate. Asmall amount of argon or nitrogen may be allowed to flow into thefurnace, however, it is preferable not to substitute the atmospherecompletely with argon or nitrogen but to adjust the oxygen concentrationin the vicinity of the surface of the material to be graphitized (within5 cm) to 1% or more, preferably 1 to 5% in the graphitization process.As an oxygen-containing gas, air is preferable but a low-oxygen gas inwhich the oxygen concentration is lowered to the above-mentioned levelmay be used as well. Using argon and nitrogen in a large amount requiresenergy for condensing the gas, and if the gas is circulated, the heatrequired for the graphitization is to be exhausted out of the system andfurther energy is to be required. From this viewpoint, it is preferableto perform the graphitization in an environment open to the atmosphere.

However, when the graphitization is carried out as described above, animpurity component derived from the material to be graphitized is likelyto precipitate in the region being in contact with oxygen, and it isdesirable to remove it. Examples of the method for removing the impurityinclude a method of removing the above-mentioned material in the regionfrom the position being in contact with an oxygen-containing air to apredetermined depth. That is, the graphite material underlying deeperthan the above position is obtained. A determined depth is 2 cm,preferably 3 cm and more preferably 5 cm from the surface.

The material underlying deeper has few chances to be in contact withoxygen. It is preferable to obtain a graphite material within 2 m fromthe portion being contact with the oxygen-containing gas, morepreferably within 1 m, and more preferably within 50 cm.

In the present invention, the carbon material can be used withoutperforming pulverizing treatment after mixing a defect repairingmaterial and carbonizing the mixture. However, the particle size may bereduced by performing the pulverizing treatment. In this case, the edgesurface of the calcined coke subjected to heat treatment after beingpulverized is to be exposed on the edge surface after pulverization.However, it has less harmful effect on the battery material when thecoke is used as a negative electrode material since the edge surface isgenerated by being broken after graphitization as heat treatment.

In the case where a part of carbon fiber is allowed to adhere to thesurface of the carbon material, the adhesion method is not particularlylimited. Examples of the methods include a method of mixing the obtainedcarbon material and carbon fiber by a mechanochemical method with aMechanofusion produced by Hosokawa Micron Corporation, and a method ofmixing carbon fiber into the pulverized calcined coke and pulverizeddefect repairing material to be well dispersed and next subjected tographitization treatment.

(3) Carbon Material for Battery Electrodes

The carbon material for battery electrodes in a preferred embodiment ofthe present invention contains the above-mentioned carbon material. Whenthe above-mentioned carbon material is used as a battery electrode,battery electrode having a high energy density can be obtained, whilemaintaining a high capacity, a high coulomb efficiency and high cyclecharacteristics.

The carbon material for a battery electrode may be used as, for example,a negative electrode active material and an agent for impartingconductivity to a negative electrode of a lithium ion secondary battery.

The carbon material for battery electrodes in a preferred embodiment ofthe present invention may comprise the above-mentioned carbon materialonly. It is also possible to use the materials obtained by blendingspherical natural or artificial graphite having d002 of 0.3370 nm orless in an amount of 0.01 to 200 parts by mass and preferably 0.01 to100 parts by mass; or by blending natural or artificial graphite (forexample, graphite having a scale shape) having d002 of 0.3370 nm or lessand aspect ratio of 2 to 100 in an amount of 0.01 to 120 parts by massand preferably 0.01 to 100 parts by mass based on 100 parts by mass ofthe carbon material. By using the graphite material mixed with othergraphite materials, the graphite material can be added with excellentproperties of other graphite materials while maintaining the excellentcharacteristics of the carbon material in a preferred embodiment of thepresent invention. With respect to mixing of these materials, theblending amount can be determined by appropriately selecting thematerials to be mixed depending on the required battery characteristics.

Carbon fiber may also be mixed with the material for battery electrodes.As the carbon fiber, carbon fiber similar to the carbon fiber describedabove may be used. The mixing amount is 0.01 to 20 parts by mass,preferably 0.5 to 5 parts by mass in terms of total 100 parts by mass ofthe above-mentioned graphite material.

Also, metal particles may be mixed with the material for batteryelectrodes. The mixing amount is 0.1 to 50 parts by mass, preferably 10to 30 parts by mass in terms of total 100 parts by mass of theabove-mentioned graphite material.

(4) Paste for Electrodes

The paste for an electrode in a preferred embodiment of the presentinvention contains the above-mentioned carbon material for a batteryelectrode and a binder. The paste for an electrode can be obtained bykneading the carbon material for a battery electrode with a binder. Aknown device such as a ribbon mixer, a screw-type kneader, a SpartanGranulator, a Loedige Mixer, a planetary mixer, or a universal mixer maybe used for kneading. The paste for an electrode may be formed into asheet shape, a pellet shape, or the like.

Examples of the binder to be used for the paste for an electrode includeknown binders such as: fluorine-based polymers such as polyvinylidenefluoride and polytetrafluoroethylene; and rubber-based binders such asstyrene-butadiene rubber (SBR).

The appropriate use amount of the binder is 1 to 30 parts by mass interms of 100 parts by mass of the material for a battery electrode, andin particular, the use amount is preferably about 3 to 20 parts by mass.

A solvent can be used at a time of kneading. Examples of the solventinclude known solvents suitable for the respective binders such as:toluene and N-methylpyrolidone in the case of a fluorine-based polymer;water in the case of SBR; dimethylformamide; isopropanol and the like.In the case of the binder using water as a solvent, it is preferred touse a thickener together. The amount of the solvent is adjusted so as toobtain a viscosity at which a paste can be applied to a currentcollector easily.

(5) Electrode

An electrode in a preferred embodiment of the present inventioncomprises a formed body of the above-mentioned paste for an electrode.The electrode is obtained, for example, by applying the paste for anelectrode to a current collector, followed by drying and pressureforming.

Examples of the current collector include foils and mesh of aluminum,nickel, copper, stainless steel and the like. The coating thickness ofthe paste is generally 50 to 200 μm. When the coating thickness becomestoo large, a negative electrode may not be accommodated in astandardized battery container. There is no particular limitation to thepaste coating method, and an example of the coating method includes amethod involving coating with a doctor blade or a bar coater, followedby molding with roll pressing or the like.

Examples of the pressure molding include roll pressing, plate pressing,and the like. The pressure for the pressure forming is preferably about1 to 3 t/cm². As the electrode density of the electrode increases, thebattery capacity per volume generally increases. However, if theelectrode density is increased too much, the cycle characteristic isgenerally degraded. If the paste for an electrode in a preferredembodiment of the present invention is used, the degradation in thecycle characteristic is small even when the electrode density isincreased. Therefore, an electrode having the high electrode density canbe obtained. The maximum value of the electrode density of the electrodeobtained using the paste for an electrode in a preferred embodiment ofthe present invention is generally 1.7 to 1.9 g/cm³. The electrode thusobtained is suitable for a negative electrode of a battery, inparticular, a negative electrode of a secondary battery.

(6) Battery, Secondary Battery

A battery or a secondary battery can be produced, using theabove-mentioned electrode as a constituent element (preferably, as anegative electrode).

The battery or secondary battery in a preferred embodiment of thepresent invention is described by taking a lithium ion secondary batteryas a specific example. The lithium ion secondary battery has a structurein which a positive electrode and a negative electrode are soaked in anelectrolytic solution or an electrolyte. As the negative electrode, theelectrode in a preferred embodiment of the present invention is used.

In the positive electrode of the lithium ion secondary battery, atransition metal oxide containing lithium is generally used as apositive electrode active material, and preferably, an oxide mainlycontaining lithium and at least one kind of transition metal elementselected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Mo, andW, which is a compound having an atom ratio of lithium to a transitionmetal element of 0.3 to 2.2, is used. More preferably, an oxide mainlycontaining lithium and at least one kind of transition metal elementselected from the group consisting of V, Cr, Mn, Fe, Co and Ni, which isa compound having an atom ratio of lithium to a transition metal elementof 0.3 to 2.2, is used. It should be noted that Al, Ga, In, Ge, Sn, Pb,Sb, Bi, Si, P, B, and the like may be contained in a range of less than30% by mole with respect to the mainly present transition metal. Of theabove-mentioned positive electrode active materials, it is preferredthat at least one kind of material having a spinel structure representedby a general formula Li_(x)MO₂ (M represents at least one kind of Co,Ni, Fe, and Mn, and x is 0 to 1.2), or Li_(y)N₂O₄ (N contains at leastMn, and y is 0 to 2) be used.

Further, as the positive electrode active material, there may beparticularly preferably used at least one kind of materials eachincluding Li_(y)M_(a)D_(1-a)O₂ (M represents at least one kind of Co,Ni, Fe, and Mn, D represents at least one kind of Co, Ni, Fe, Mn, Al,Zn, Cu, Mo, Ag, W, Ga, In, Sn, Pb, Sb, Sr, B, and P with the provisothat the element corresponding to M being excluded, y=0 to 1.2, anda=0.5 to 1) or materials each having a spinel structure represented byLi_(z) (N_(b)E_(1-b))₂O₄ (N represents Mn, E represents at least onekind of Co, Ni, Fe, Mn, Al, Zn, Cu, Mo, Ag, W, Ga, In, Sn, Pb, Sb, Sr, Band P, b=1 to 0.2, and z=0 to 2).

Specifically, there are exemplified Li_(x)CoO₂, Li_(x)NiO₂, Li_(x)MnO₂,Li_(x)Co_(a)Ni_(1-a)O₂, Li_(x)Co_(b)V_(1-b)Oz, Li_(x)Co_(b)Fe_(1-b)O₂,Li_(x)Mn₂O₄, Li_(x)Mn_(c)Co_(2-c)O₄, Li_(x)Mn_(c)Ni_(2-c)O₄,Li_(x)Mn_(c)V_(2-c)O₄, and Li_(x)Mn_(c)Fe_(2-c)O₄ (where, x=0.02 to 1.2,a=0.1 to 0.9, b=0.8 to 0.98, c=1.6 to 1.96, and z=2.01 to 2.3). As themost preferred transition metal oxide containing lithium, there aregiven Li_(x)CoO₂, Li_(x)NiO₂, Li_(x)MnO₂, Li_(x)Co_(a)Ni_(1 a)O₂,Li_(x)Mn₂O₄, and Li_(x)Co_(b)V_(1 b)O_(z) (x=0.02 to 1.2, a=0.1 to 0.9,b=0.9 to 0.98, and z=2.01 to 2.3). It should be noted that the value ofx is a value before starting charge and discharge, and the valueincreases and decreases in accordance with charge and discharge.

Although the average particle size of the positive electrode activematerial is not particularly limited, the size is preferably 0.1 to 50μm. It is preferred that the volume of the particles of 0.5 to 30 μm be95% or more. It is more preferred that the volume occupied by theparticle group with a particle diameter of 3 μm or less be 18% or lessof the total volume, and the volume occupied by the particle group of 15μm or more and 25 μm or less be 18% or less of the total volume.

Although the specific area is not particularly limited, the area ispreferably 0.01 to 50 m²/g, particularly preferably 0.2 m²/g to 1 m²/gby a BET method. Further, it is preferred that the pH of a supernatantobtained when 5 g of the positive electrode active material is dissolvedin 100 ml of distilled water be 7 or more and 12 or less.

In a lithium ion secondary battery, a separator may be provided betweena positive electrode and a negative electrode. Examples of the separatorinclude non-woven fabric, cloth, and a microporous film each mainlycontaining polyolefin such as polyethylene and polypropylene, acombination thereof, and the like.

As an electrolytic solution and an electrolyte forming the lithium ionsecondary battery in a preferred embodiment of the present invention, aknown organic electrolytic solution, inorganic solid electrolyte, andpolymer solid electrolyte may be used, but an organic electrolyticsolution is preferred in terms of electric conductivity.

As an organic electrolytic solution (non-aqueous solvent), preferred isa solution of an organic solvent such as: an ether such as diethylether, dibutyl ether, ethylene glycol monomethyl ether, ethylene glycolmonoethyl ether, ethylene glycol monobutyl ether, diethylene glycolmonomethyl ether, diethylene glycol monoethyl ether, diethylene glycolmonobutyl ether, diethylene glycol dimethyl ether, ethylene glycolphenyl ether, or 1,2-dimethoxyethane; an amide such as formamide,N-methylformamide, N,N-dimethylformamide, N-ethylformamide,N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide,N-ethylacetamide, N,N-diethylacetamide, N,N-dimethylpropionamide, orhexamethylphosphorylamide; a sulfur-containing compound such asdimethylsulfoxide or sulfolane; a dialkyl ketone such as methyl ethylketone or methyl isobutyl ketone; a cyclic ether such as ethylene oxide,propylene oxide, tetrahydrofuran, 2-methoxytetrahydrofuran, or1,3-dioxolan; a carbonate such as ethylene carbonate or propylenecarbonate; γ-butyrolactone; N-methylpyrrolidone; acetonitrile;nitromethane; or the like. There are more preferably exemplified: esterssuch as ethylene carbonate, butylene carbonate, diethyl carbonate,dimethyl carbonate, propylene carbonate, vinylene carbonate, andγ-butyrolactone; ethers such as dioxolan, diethyl ether, anddiethoxyethane; dimethylsulfoxide; acetonitrile; tetrahydrofuran; andthe like. A carbonate-based nonaqueous solvent such as ethylenecarbonate or propylene carbonate may be particularly preferably used.One kind of those solvents may be used alone, or two or more kindsthereof may be used as a mixture.

A lithium salt is used for a solute (electrolyte) of each of thosesolvents. Examples of a generally known lithium salt include LiClO₄,LiBF₄, LiPF₆, LiAlCl₄, LiSbF₆, LiSCN, LiCl, LiCF₃SO₃, LiCF₃CO₂,LiN(CF₃SO₂)₂, and the like.

Examples of the polymer solid electrolyte include a polyethylene oxidederivative and a polymer containing the derivative, a polypropyleneoxide derivative and a polymer containing the derivative, a phosphoricacid ester polymer, a polycarbonate derivative and a polymer containingthe derivative, and the like.

It should be noted that there is no constraint for the selection ofmembers required for the battery configuration other than theaforementioned members.

EXAMPLES

Hereinafter, the present invention is described in more detail by way oftypical examples. It should be noted that these examples are merely forillustrative purposes, and the present invention is not limited thereto.

It should be noted that, as for the carbon materials of Examples andComparative Examples, observation and data analysis with respect tooptical structures, average interplanar spacing (d002) by an X-raydiffraction method, R values, and BET specific surface area are measuredby the method described in detail in the “Mode for carrying out theInvention” of the specification. Further, the methods for measuringother physical properties are given below.

(1) Average Particle Diameter (D50)

The average particle diameter based on a volume (D50) was determinedusing Mastersizer produced by Malvern Instruments Ltd. as a laserdiffraction type measurement device of particle size distribution.

(2) Method for Evaluating Batteries

-   -   a) Production of Paste:

Styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) wereprepared. The SBR powder was dispersed in purified water to therebyobtain an SBR dispersion. Also, the CMC powder was mixed with purifiedwater and stirred with a stirrer all day and all night to thereby obtaina CMC solution.

Carbon black was prepared as a conductive assistant. 95 parts by mass ofa material for a battery electrode, 2 parts by mass of a conductiveassistant, a CMC solution containing 1.5 parts by mass of a solidcomponent and an SBR dispersion containing 1.5 parts by mass of a solidcomponent were mixed, and water was appropriately added thereto foradjusting the viscosity. The mixture was kneaded by a planetary andcentrifugal mixer to thereby obtain a paste for a negative electrode.

-   -   b) Production of an Electrode:

The paste for a negative electrode is applied to a high-purity copperfoil to a thickness of 250 μm using a doctor blade. The high-puritycopper foil thus obtained is dried in vacuum at 120° C. for 1 hour andpunched into a size of 18 mmΦ. The electrode thus punched out issandwiched between pressing plates made of super-steel and pressed sothat a press pressure becomes about 1×10² to 3×10² N/mm² (1×10³ to 3×10³kg/cm²) with respect to the electrode. Then, the electrode is dried in avacuum drier at 120° C. for 12 hours to obtain an electrode forevaluation.

-   -   c) Production of a Battery:

A three-electrode cell is produced as follows. The following operationis performed in a dry argon atmosphere at a dew point of −80° C. orless.

In a cell (inner diameter: about 18 mm) with a screwed-type lid made ofpolypropylene, the carbon electrode with a copper foil produced in theabove-mentioned item (2) and a metal lithium foil are sandwiched betweenseparators (microporous films made of polypropylene (Cell Guard 2400))and stacked. Further, metal lithium for reference is stacked in the sameway. An electrolyte is added to the resultant to obtain a cell fortesting.

-   -   d) Electrolyte:

In a mixed solution of 8 parts by mass of ethylene carbonate (EC) and 12parts by mass of diethyl carbonate (DEC), 1 mol/liter of LiPF₆ isdissolved as an electrolyte.

-   -   e) Initial Charge and Discharge Efficiency and a Discharge        Capacity:

Regarding charging (insertion of lithium into carbon), constant current(CC) charging is performed at 0.2 mA/cm² from a rest potential to 0.002V. Next, the charging is switched to constant voltage (CV) charging at0.002 V and stopped when a current value drops to 25.4 μA.

A constant-current and constant-voltage discharge test is performed at acurrent density of 0.4 mA/cm² (corresponding to 0.2 C) and 10 mA/cm²(corresponding to 5 C). The test is performed in a thermostatic chamberat 25° C. At that time, the ratio of the electricity of the initialcharge and discharge, i.e. discharge electricity/charge electricity inpercentage was defined as an index of the initial charge and dischargeefficiency.

The discharge capacity was calculated by dividing the dischargeelectricity at 0.4 mA/cm² (corresponding to 0.2 C) by the activesubstance mass per unit area.

-   -   f) Charge/Discharge Cycle Capacity Keeping Ratio (150 Cycles):

A constant-current and constant-voltage charge/discharge test isperformed at a current density of 2 mA/cm² (corresponding to 1 C).

Regarding charging (insertion of lithium into carbon), constant current(CC) charging is performed at 0.2 mA/cm² from a rest potential to 0.002V. Next, the charging is switched to constant voltage (CV) charging at0.002 V and stopped when a current value drops to 25.4 μA.

Regarding discharging (discharge from carbon), CC discharging isperformed at a predetermined current density and cut off at a voltage of1.5 V. Further, the measurement is performed in a thermostatic chamberat 60° C., and charge/discharge is repeated 150 cycles.

-   -   g) Capacity Ratio of the Rate Characteristics Test at a Low        Temperature:

Charge was performed at 25° C. under the above-mentioned conditions.Discharge was performed in a thermostatic chamber at −20° C. Dischargecapacity at −20° C./discharge capacity at 25° C. was calculated anddefined as an index of the capacity ratio of the rate characteristicstest at a low temperature.

Example 1

A crude oil produced in Liaoning, China (28° API, wax content of 17% andsulfur content of 0.66%) was distilled under ordinary pressure. Using aY-type zeolite catalyst in a sufficient amount against the heavydistillate, catalytic cracking in a fluidized bed was performed at 510°C. under ordinary pressure. A solid content such as a catalyst wascentrifuged until the obtained oil became clear to thereby obtain decantoil 1. The oil was subjected to a small-sized delayed coking process.After keeping the drum inlet temperature at 505° C. and the druminternal pressure to 600 kPa (6 kgf/cm²) for ten hours, the drum waswater-cooled to obtain black chunks. After pulverizing the obtainedblack chunks into pieces up to five centimeter in size with a hammer,they were heated in a rotary kiln (external-heating type with anelectrical heater; aluminum oxide SSA-S; Φ120 mm inner tube) in whichthe outer wall temperature in the center of the inner tube is set at1,450° C. by adjusting the feeding rate and tilting angle of the blackchunks so as to set the retention time to 15 minutes.

The obtained red-hot sample was cooled by water-cooling the outsidethereof in an SUS container, while it was sealed from the air andnitrogen as required was introduced so that the inside the container isnot subjected to negative pressure. A black and slightly gray blocksample up to 2 cm in size was obtained as calcined coke 1.

Calcined coke 1 was observed under a polarizing microscope for the imageanalysis. As a result of the measurement, when areas of the opticalstructures are accumulated from the smallest structure in an ascendingorder, an area of a structure whose accumulated area corresponds to 60%of the total area was 47.4 μm². When the detected particles are arrangedfrom the particle of the smallest aspect ratio in an ascending order,the aspect ratio of the particle which ranks at the position of 60% inthe total number of all the particles was 2.66.

Calcined coke 1 was pulverized with a bantam mill produced by HosokawaMicron Corporation and subsequently coarse powder was excluded with asieve having a mesh size of 32 μm. Next, the pulverized coke issubjected to air-flow screening with Turboclassifier TC-15N produced byNisshin Engineering Inc. to obtain a powder calcined coke 1, wherein D50is 19.3 μm, substantially containing no particles each having a particlediameter of 1.0 μm or less.

A graphite crucible was filled with calcined coke 1 and placed in anAcheson furnace with a carbonized carbon felt (2 mm) was softly put onthe crucible to prevent a rapid inflow of the air. After the heattreatment at 3,150° C., the mixture was mixed well to be used as asample, thereby obtaining graphite 1.

85 g of the obtained graphite 1 was mixed with 15 g of isotropic coalpitch (softening point of 130° C., remaining coal rate of 60%) having aD50 value of 3.1 μm and substantially containing no particles eachhaving a particle diameter of 20 μm or more by a dry method with aplanetary and centrifugal mixer at 2,000 rpm for 20 minutes to obtain amixture. The mixture was calcined under nitrogen atmosphere for one hourin a siliconit furnace maintained at 1,100° C.

After measuring the various physical properties of the obtained carbonmaterial, an electrode was produced as described above and the cyclecharacteristics and the like were measured. Table 2 shows the results.

Example 2

Coal tar derived from bituminous coal was distilled at 320° C. underordinary pressure and distillate of the distillation temperature orlower was removed. From the obtained tar having a softening point of 30°C., the insoluble matter was removed by filtration at 100° C. to obtainviscous liquid 1. The liquid was subjected to a small-sized delayedcoking process. After keeping the drum inlet temperature at 510° C. andthe drum internal pressure to 500 kPa (5 kgf/cm²) for ten hours, thedrum was water-cooled to obtain black chunks. After pulverizing theobtained black chunks into pieces up to five centimeter in size with ahammer, they were heated in a rotary kiln (external-heating type with anelectrical heater; aluminum oxide SSA-S; Φ120 mm inner tube) in whichthe outer wall temperature in the center of the inner tube is set at1,450° C. by adjusting the feeding rate and tilting angle of the blackchunks so as to set the retention time to 15 minutes.

The obtained red-hot sample was cooled in an SUS container in a similarmanner as in Example 1 to obtain black block sample up to 3 cm in sizeas calcined coke 2.

Calcined coke 2 was observed under a polarizing microscope for the imageanalysis in the same way as in Example 1. Table 2 shows the results.

Calcined coke 2 was pulverized in a similar manner as in Example 1 toobtain a powder calcined coke 2. A graphite crucible was filled withpowder calcined coke 2 and placed in an Acheson furnace with acarbonized carbon felt (2 mm) was softly put on the crucible to preventa rapid inflow of the air. After the heat treatment at 3,150° C., themixture was mixed well to be used as a sample, thereby obtaininggraphite 2.

80 g of the obtained graphite 2 was mixed with 20 g of isotropicpetroleum pitch (softening point of 230° C., remaining coal rate of 68%)having a D50 value of 2.8 μm and substantially containing no particleseach having a particle diameter of 20 μm or more by a dry method with aplanetary and centrifugal mixer at 2,000 rpm for 20 minutes to obtain amixture. The mixture was calcined under nitrogen atmosphere for one hourin a siliconit furnace maintained at 1,000° C. The obtained sample waspulverized with Wonder Blender (manufactured by OSAKA CHEMICAL Co.,Ltd.) for two minutes to obtain a carbon material.

After measuring the various physical properties of the obtained carbonmaterial, an electrode was produced in the same way as in Example 1 andthe cycle characteristics and the like were measured. Table 2 shows theresults.

Example 3

An Iranian crude oil (30° API, wax content of 2% and sulfur content of0.7%) was distilled under ordinary pressure. Using a Y-type zeolitecatalyst in a sufficient amount against the heavy distillate, catalyticcracking in a fluidizing bed was performed at 500° C. under ordinarypressure. A solid content such as a catalyst was centrifuged until theobtained oil became clear to thereby obtain decant oil 2. The oil wassubjected to a small-sized delayed coking process. After keeping thedrum inlet temperature at 550° C. and the drum internal pressure to 600kPa (6 kgf/cm²) for ten hours, the drum was water-cooled to obtain blackchunks. After pulverizing the obtained black chunks into pieces having amaximum size of about five centimeters with a hammer, they were heatedin a rotary kiln (external-heating type with an electrical heater;aluminum oxide SSA-S; Φ120 mm inner tube) in which the outer walltemperature in the center of the inner tube is set at 1,450° C. byadjusting the feeding rate and tilting angle of the black chunks so asto set the retention time to 15 minutes.

The obtained red-hot sample was cooled in an SUS container in a similarmanner as in Example 1 to obtain black and slightly gray block sample upto 2 cm in size as calcined coke 3.

Calcined coke 3 was observed under a polarizing microscope for the imageanalysis in the same way as in Example 1. Table 2 shows the results.

Calcined coke 3 was pulverized in a similar manner as in Example 1 toobtain a powder calcined coke 3. A graphite crucible was filled withcalcined coke 3 and placed in an Acheson furnace with a carbonizedcarbon felt (2 mm) was softly put on the crucible to prevent a rapidinflow of the air. After the heat treatment at 3,150° C., the mixturewas mixed well to be used as a sample, thereby obtaining graphite 3.

70 g of the obtained graphite 3 was mixed with 30 g of anisotropic coalpitch (softening point of 126° C., remaining coal rate of 61%) having aD50 value of 2.7 μm and substantially containing no particles eachhaving a particle diameter of 20 μm or more by a dry method with aplanetary and centrifugal mixer at 2,000 rpm for 20 minutes to obtain amixture. The mixture was calcined under nitrogen atmosphere for one hourin a siliconit furnace maintained at 900° C. The obtained sample waspulverized with Wonder Blender (manufactured by OSAKA CHEMICAL Co.,Ltd.) for two minutes to obtain a carbon material.

After measuring the various physical properties of the obtained carbonmaterial, an electrode was produced in the same way as in Example 1 andthe cycle characteristics and the like were measured. Table 2 shows theresults.

Example 4

Decant oil in Example 1 and viscous liquid 1 in Example 2 were subjectedto in-line mixing at a one-to-one rate on a volume basis while keepingthe pipe hot. The oil was subjected to a small-sized delayed cokingprocess. After keeping the drum inlet temperature at 505° C. and thedrum internal pressure to 600 kPa (6 kgf/cm²) for ten hours, the drumwas water-cooled to obtain black chunks. After pulverizing the obtainedblack chunks into pieces having a maximum size of about five centimeterswith a hammer, they were heated in a rotary kiln (external-heating typewith an electrical heater; aluminum oxide SSA-S; Φ120 mm inner tube) inwhich the outer wall temperature in the center of the inner tube is setat 1,450° C. by adjusting the feeding rate and tilting angle of theblack chunks so as to set the retention time to 15 minutes.

The obtained red-hot sample was cooled in an SUS container in a similarmanner as in Example 1 to obtain black and slightly gray block sample upto 2 cm in size as calcined coke 4.

Calcined coke 4 was observed under a polarizing microscope for the imageanalysis in the same way as in Example 1. Table 2 shows the results.

Calcined coke 4 was pulverized in a similar manner as in Example 1 toobtain a powder calcined coke 4. A graphite crucible was filled with thepowder calcined coke 4 and placed in an Acheson furnace with acarbonized carbon felt (2 mm) was softly put on the crucible to preventa rapid inflow of the air. After the heat treatment at 3,150° C., themixture was mixed well to be used as a sample, thereby obtaininggraphite 4.

80 g of the obtained graphite 4 was mixed with 20 g of anisotropicpetroleum pitch (softening point of 126° C., remaining coal rate of 61%)having a D50 value of 2.0 μm and substantially containing no particleseach having a particle diameter of 20 μm or more by a dry method with aplanetary and centrifugal mixer at 2,000 rpm for 20 minutes to obtain amixture. The mixture was calcined under nitrogen atmosphere for one hourin a siliconit furnace maintained at 1,200° C. The obtained sample waspulverized with Wonder Blender (manufactured by OSAKA CHEMICAL Co.,Ltd.) for two minutes to obtain a carbon material.

After measuring the various physical properties of the obtained carbonmaterial, an electrode was produced in the same way as in Example 1 andthe cycle characteristics and the like were measured. Table 2 shows theresults.

Example 5

Decant oil 2 in Example 3 and viscous liquid 1 in Example 2 weresubjected to in-line mixing at a one-to-one rate on a volume basis whilekeeping the pipe hot. The oil was subjected to a small-sized delayedcoking process. After keeping the drum inlet temperature at 505° C. andthe drum internal pressure to 600 kPa (6 kgf/cm²) for ten hours, thedrum was water-cooled to obtain black chunks. After pulverizing theobtained black chunks into pieces having a maximum size of about fivecentimeters with a hammer, they were heated in a rotary kiln(external-heating type with an electrical heater; aluminum oxide SSA-S;Φ120 mm inner tube) in which the outer wall temperature in the center ofthe inner tube is set at 1,450° C. by adjusting the feeding rate andtilting angle of the black chunks so as to set the retention time to 15minutes.

The obtained red-hot sample in an SUS container was sealed from the airand cooled by water-cooling the outside of the container, while nitrogenas needed was introduced so that the inside the container is notsubjected to negative pressure, to thereby obtain black and slightlygray block sample up to 2 cm in size as calcined coke 5.

Calcined coke 5 was observed under a polarizing microscope for the imageanalysis in the same way as in Example 1. Table 2 shows the results.

Calcined coke 5 was pulverized in a similar manner as in Example 1 toobtain a powder calcined coke 5. A graphite crucible was filled with thepowder calcined coke 5 and placed in an Acheson furnace with acarbonized carbon felt (2 mm) was softly put on the crucible to preventa rapid inflow of the air. After the heat treatment at 3,150° C., themixture was mixed well to be used as a sample, thereby obtaininggraphite 5.

80 g of the obtained graphite 5 was mixed with 20 g of isotropic coalpitch (softening point of 120° C., remaining coal rate of 59%) having aD50 value of 2.5 μm and substantially containing no particles eachhaving a particle diameter of 20 μm or more by a dry method with aplanetary and centrifugal mixer at 2,000 rpm for 20 minutes to obtain amixture. The mixture was calcined under nitrogen atmosphere for one hourin a siliconit furnace maintained at 1,250° C. The obtained sample waspulverized with Wonder Blender (manufactured by OSAKA CHEMICAL Co.,Ltd.) for two minutes to obtain a carbon material.

After measuring the various physical properties of the obtained carbonmaterial, an electrode was produced in the same way as in Example 1 andthe cycle characteristics and the like were measured. Table 2 shows theresults.

Example 6

Decant oil 1 in Example 1 and viscous liquid 1 in Example 2 weresubjected to in-line mixing at a one-to-one rate on a volume basis whilekeeping the pipe hot. The oil was subjected to a small-sized delayedcoking process. After keeping the drum inlet temperature at 505° C. andthe drum internal pressure to 600 kPa (6 kgf/cm²) for ten hours, thedrum was water-cooled to obtain black chunks. After pulverizing theobtained black chunks into pieces having a maximum size of about fivecentimeters with a hammer, they were heated in a rotary kiln(external-heating type with an electrical heater; aluminum oxide SSA-S;Φ120 mm inner tube) in which the outer wall temperature in the center ofthe inner tube is set at 1,450° C. by adjusting the feeding rate andtilting angle of the black chunks so as to set the retention time to 15minutes.

The obtained red-hot sample in an SUS container was sealed from the airand cooled by water-cooling the outside of the container, while nitrogenas needed was introduced so that the inside the container is notsubjected to negative pressure, to thereby obtain black and slightlygray block sample up to 2 cm in size as calcined coke 6.

Calcined coke 6 was observed under a polarizing microscope for the imageanalysis in the same way as in Example 1. Table 2 shows the results.

Calcined coke 6 was pulverized in a similar manner as in Example 1 toobtain a powder calcined coke 6. A graphite crucible was filled with thepowder calcined coke 6 and placed in an Acheson furnace with acarbonized carbon felt (2 mm) was softly put on the crucible to preventa rapid inflow of the air. After the heat treatment at 3,150° C., themixture was mixed well to be used as a sample, thereby obtaininggraphite 6.

80 g of the obtained graphite 6 was mixed with 20 g of isotropicpetroleum pitch (softening point of 120° C., remaining coal rate of 59%)having a D50 value of 6.2 μm and substantially containing no particleseach having a particle diameter of 30 μm or more by a dry method with aplanetary and centrifugal mixer at 2,000 rpm for 20 minutes to obtain amixture.

The mixture was calcined under nitrogen atmosphere for one hour in asiliconit furnace maintained at 1,100° C. The obtained sample waspulverized with Wonder Blender (manufactured by OSAKA CHEMICAL Co.,Ltd.) for two minutes to obtain a carbon material.

After measuring the various physical properties of the obtained carbonmaterial, an electrode was produced in the same way as in Example 1 andthe cycle characteristics and the like were measured. Table 2 shows theresults.

Comparative Example 1

After subjecting powder calcined coke 2 in Example 2 was subjected toheat treatment at 3,150° C. in an Acheson furnace in the same way as inExample 1, the powder was mixed well to be used as a sample.

After measuring the various physical properties of the obtained carbonmaterial, an electrode was produced in the same way as in Example 1 andthe cycle characteristics and the like were measured. Table 2 shows theresults.

Comparative Example 2

Residue obtained by distilling crude oil produced in the West Coastunder reduced pressure was used as a raw material. The properties of thematerial are 18° API, wax content of 11 mass % and sulfur content of 3.5mass %. The material was subjected to a small-sized delayed cokingprocess. After keeping the drum inlet temperature at 490° C. and thedrum internal pressure to 2 kgf/cm² for ten hours, the drum waswater-cooled to obtain black chunks. After pulverizing the obtainedblack chunks into pieces having a maximum size of about five centimeterswith a hammer, they were heated in a rotary kiln (external-heating typewith an electrical heater; aluminum oxide SSA-S; Φ120 mm inner tube) inwhich the outer wall temperature in the center of the inner tube is setat 1,450° C. by adjusting the feeding rate and tilting angle of theblack chunks so as to set the retention time to 15 minutes.

The obtained red-hot sample was cooled in an SUS container in a similarmanner as in Example 1 to obtain black and slightly gray block sample upto 3 cm in size as calcined coke 7.

Calcined coke 7 was observed under a polarizing microscope for the imageanalysis in the same way as in Example 1. Table 2 shows the results.

Calcined coke 7 was pulverized in a similar manner as in Example 1 toobtain a powder calcined coke 7. 98 g of the obtained powder calcinedcoke 7 was mixed with 2 g of isotropic petroleum pitch (softening pointof 120° C., remaining coal rate of 59%) having a D50 value of 2.8 μm andsubstantially containing no particles each having a particle diameter of20 μm or more by a dry method with a planetary and centrifugal mixer at2,000 rpm for 20 minutes to obtain a mixture.

A graphite crucible was filled with the mixture and placed in an Achesonfurnace with a carbonized carbon felt (2 mm) was softly put on thecrucible to prevent a rapid inflow of the air. After the heat treatmentat 3,150° C., the mixture was mixed well to be used as a sample.

After measuring the various physical properties of the obtained carbonmaterial, an electrode was produced in the same way as in Example 1 andthe cycle characteristics and the like were measured. Table 2 shows theresults.

In this example, the volume energy density of the electrode is low,which causes disadvantage in obtaining a high-density battery.

Comparative Example 3

After measuring the various physical properties of SFG44 produced byTIMCAL Graphite & Carbon, an electrode was produced in the same way asin Example 1 and the cycle characteristics and the like were measured.Table 2 shows the results.

In this example the capacity retention ratio of the electrode is low,which causes disadvantage in obtaining a high-density battery.

Comparative Example 4

100 g of Chinese scale-like natural graphite (solid carbon content: 99%,specific surface area: 9.1 m²/g, D50: 26.8 μm) was processed withHybridization System NHS-1 produced by Nara Machinery Co., Ltd. at arotation speed of 50 m/s for three minutes. The treatment was repeateduntil the sample amount reaches 3.6 kg. After adding 0.4 kg of petroleumpitch pulverized so as to have D50 of 6 μm, the mixture was put into aLoedige Mixer produced by MATSUBO Corporation, and mixed until itbecomes uniform by visual observation. Subsequently, 200 g of themixture was put in an alumina crucible and heated to 1,300° C. undernitrogen atmosphere and maintained at the temperature for two hours. Theobtained heat-treated product was pulverized with a pin mill, andparticles having a size of 2 μm or less and particles having a size of45 μm or more were classified and removed until they are notsubstantially observed in the product by a particle size distributionanalyzer. After measuring the various physical properties of theproduct, an electrode was produced in the same way as in Example 1 andthe cycle characteristics and the like were measured. Table 2 shows theresults.

In this example the capacity retention ratio of the electrode is low,which causes disadvantage in obtaining a high-density battery.

TABLE 2 Com- Com- Com- Com- parative parative parative parative Ex. 1Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Area (SOP) μm²12.32 13.12 7.84 8.16 7.68 8.48 14.08 6.24 28.96 99.72 Aspect ratio —2.14 2.18 2.14 2.18 2.15 2.17 2.19 1.93 2.28 2.16 (AROP) Averageparticle μm 19.00 13.20 9.00 10.90 14.70 10.50 10.60 20.80 24.80 15.4diameter (D50) (SOP * AROP)^(1/2)/ — 0.27 0.41 0.46 0.39 0.28 0.41 0.520.17 0.33 0.95 D50 d002 nm 0.3356 0.3356 0.3357 0.3355 0.3357 0.33560.3355 0.3362 0.3354 0.3356 R value — 0.7 0.4 0.5 0.5 0.4 0.4 0.0 0.40.1 0.2 BET specific m²/g 4.3 6.7 9.1 6.6 7.4 6.1 3.6 5.3 4.6 1.8surface area Area (calcined μm² 47.4 110.6 74.7 82.7 96.2 82.7 110.6 6.4— — coke before pulverizing) Aspect ratio — 2.7 2.8 2.5 2.7 2.7 2.7 2.81.9 — — (calcined coke before pulverizing) Average particle μm 19.3 21.018.0 15.8 23.5 23.1 21.0 19.7 — — diameter of pulverized calcined coke(D50) Average particle μm 3.1 2.8 2.7 2 2.5 6.2 Not 2.8 — 3.2 diameterof raw used material pitch (D50) Discharge capacity mAh/cc 492 523 489522 518 510 507 408 555 554.9 density Initial charge and % 90.1 89.989.7 90.5 90.8 89.1 50.4 91.0 91.0 91.0 discharge efficiency Cyclecapacity % 85 84 86 83 82 84 36 80 66 67 retention rate (150 cycles)Capacity ratio of % 79 79 83 82 75 74 54 64 60 64 the ratecharacteristics test at a low temperature Main material — ArtificialArtificial Artificial Artificial Artificial Artificial ArtificialArtificial Artificial Natural graphite graphite graphite graphitegraphite graphite graphite graphite graphite graphite

1. A carbon material, the carbon material having a scale-like shape,wherein the ratio I_(G)/I_(D) (R value) between the peak intensity(I_(D)) of a peak in a range of 1300 to 1400 cm⁻¹ and the peak intensity(I_(G)) of a peak in a range of 1580 to 1620 cm⁻¹ measured by Ramanspectroscopy spectra when particles of the carbon material are measuredwith Raman microspectrometer is 0.38 or more and 1.2 or less and theaverage interplanar spacing d002 of plane (002) by the X-ray diffractionmethod is 0.335 nm or more and 0.338 nm or less; and by observing theoptical structures in the cross-section of the formed body made of thecarbon material in a rectangular field of 480 μm×640 μm under apolarizing microscope, when areas of the optical structures areaccumulated from a smallest structure in an ascending order, SOPrepresents an area of an optical structure whose accumulated areacorresponds to 60% of the total area of all the optical structures; whenthe structures are counted from a structure of a smallest aspect ratioin an ascending order, AROP represents the aspect ratio of the structurewhich ranks at the position of 60% in the total number of all thestructures; and when D50 represents a volume-based average particlediameter by laser diffraction method; SOP, AROP and D50 satisfy thefollowing relationship:1.5≤AROP≤6 and0.2×D50≤(SOP×AROP)^(1/2)<2×D50.
 2. The carbon material according toclaim 1, which is artificial graphite.
 3. The carbon material accordingto claim 1, wherein the carbon material has a volume-based averageparticle diameter by laser diffraction method (D50) of 1 μm or more and50 μm or less.
 4. The carbon material according to claim 1, BET specificsurface area of which is 2 m²/g or more and 25 m²/g or less.
 5. Thecarbon material according to claim 1, wherein a rhombohedral peak ratiois 1% or less.
 6. The carbon material according to claim 1, wherein apowder density (tap density) is 0.4 g/cm³ or more and 1.3 g/cm³ or less.7. A method for producing the carbon material according to claim 1,comprising a process of subjecting particles obtained by pulverizing thecalcined coke to heat treatment at a temperature of 2,400° C. or moreand 3,600° C. or less, mixing the resultant with particles obtained bypulverizing petroleum pitch or coal-tar pitch, and subjecting themixture to heat treatment at a temperature of 800° C. or more and 1,400°C. or less.
 8. The production method according to claim 7, comprisingpulverization or crushing the mixture after the process of the heattreatment of 800° C. or more and 1,400° C. or less.
 9. The productionmethod according to claim 7, wherein a volume-based average particlediameter by laser diffraction method of the particles obtained bypulverizing calcined coke (D50) Dc is 1 μm or more and 50 μm or less,and a volume-based average particle diameter of the particles obtainedby pulverizing petroleum pitch or coal-tar pitch (D50) Dp is smallerthan Dc and is 0.01 μm or more and 25 μm or less.
 10. The productionmethod according to claim 9, wherein Dc/Dp is 1.5 or more and less than200.
 11. The production method according to claim 7, wherein the mass ofthe particles obtained by pulverizing petroleum pitch or coal-tar pitchis 5 mass % or more and 35 mass % or less to the total mass of theparticles obtained by pulverizing calcined coke and the particlesobtained by pulverizing petroleum pitch or coal-tar pitch.
 12. Theproduction method according to claim 7, wherein by observing the opticalstructures of the calcined coke in a rectangular field of 480 μm×640 μmunder a polarizing microscope, when areas of the optical structures areaccumulated from a smallest structure in an ascending order, an area ofan optical structure whose accumulated area corresponds to 60% of thetotal area of all the optical structures is 10 μm² or more and 5,000 μm²or less; and when the optical structures are counted from a structure ofa smallest aspect ratio in an ascending order, the aspect ratio of thestructure which ranks at the position of 60% in the total number of allthe structures is 1.5 or more and 6 or less.
 13. A material for abattery electrode, comprising the carbon material according to claim 1.14. A material for a battery electrode, comprising 100 parts by mass ofthe carbon material according to claim 1 and 0.01 to 200 parts by massof natural graphite or artificial graphite, wherein an averageinterplanar spacing (d002) of the natural graphite or artificialgraphite is 0.3380 nm or less.
 15. A material for a battery electrode,comprising 100 parts by mass of the carbon material according to claim 1and 0.01 to 120 parts by mass of natural graphite or artificialgraphite, wherein an aspect ratio of the natural graphite or artificialgraphite is 2 to 100, and an average interplanar spacing (d002) of thenatural graphite or artificial graphite is 0.3380 nm or less.
 16. Apaste for an electrode comprising the material for a battery electrodeaccording to claim 13 and a binder.
 17. An electrode comprising a formedbody of the paste for an electrode according to claim
 16. 18. A batterycomprising the electrode according to claim 17 as a constitutingelement.